How does a permanent magnet lift iron filings

  • Thread starter sridhar_n
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  • #26
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to krab:

I finally found out what's the problem. The formulas you wrote for the energy and work are mechanical work. You still need to add the energy of the electric currents. If you do that then you'll get zero work. This is in one of Feynman's lectures, chapter 13 I think.
I'm kind of busy now but as soon as I get some spare time I'll post here the most important of the aforementioned lecture.

to rayjohn:
A dynamo does not work in the sense that it can't speed up an electron. what it does is this: it takes the work done by the waterfall or wathever and converts it into electric work. So really the one doing the work is the one moving the dynamo.
 
  • #27
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mag work.

To Ejavier
I do not understand your answer, dynamos are quite capable of moving electrons since they generate current , it's clear where the original energy came from, and it's clear that in an ideal set up the field is not dissipating energy in the form of heat but it is transmitting energy and doing so by accellerating electrons.
So I'm not really sure what you mean by not doing work, do you mean producing work as an energy source -- I would agree with that.
But if I compared to a lever lifting a weight ( a transmitter of work) would I not say that the lever is doing work ( but not producing it).
Maybe it's just semantics or there is a different definition being used, but there is no question that magnetic fields accellerate electrons without that effect we'd all fry from the sun's radiation.
Also I believe that a magnetic field can be a source of energy , if you take the case of two bar magnets attracting each other they accellerate collide and dissipate heat and sound. The energy comes from the partial collapse or reorientation of the fields.
In that case the system can be recharged by pulling them apart , but to me that's rather like recharging a battery and were quite happy saying that a battery can do work .
 
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  • #28
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I said that a magnetic field can't SPEED UP electrons and I stand by that. It can deflect them of course.

The fact is that the force that a magnetic field exerts on an electron is always perpendicular to the trajectory (same as a satellite in a circular trajectory in a central force field) so it doesn't work.

In the example you wrote about the bar magnets, there is of course a mechanical work done, but you have to take into account the work done to sustain the currents. It's all explained in the second volume of Feynman's lectures, chapter 13 I think.
 
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  • #29
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the field certainly doesnot do anywork.all that happnes is it lifts the iron(or even U may thought of attraction between two magnet),perpendicular to the line of force so that F=vxB is still vaid.In some cases induction also take place and by 3rd law of Maxwell,it is electric field which does the work
 
  • #30
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So then a 'relay' operates via an 'electric' field --- right -- and no work is done in closing it's contacts -- according to this we can do without inductors entirely, and you know you can ( in theory by using an active device -- see switched capacitor filters the phase shift required is 180 dgrees ).
An inductor is the perfect model -- it can have a permanent field and a dynamic one.
It's equivalent circuit is that of pure inductance which is an energy storage device.
It is the stored energy which can do work and if it does the energy must be replenished.
EJaviers example of a rotating vector is like a hammer hitting nothing , but we use the hammer to drive the nail , and we don't say 'my fist did it ' even tho' thats where the energy came from.
Dynamic magnetic fields can speed up electrons which is why the suns storms exist especially due to field collapse.
However if you wish to remove magnetics by talking of charge spin fine .
So according to you electromagnetic energy ( radiowaves ) is only carried by the electric field -- tell Maxwell -- the energy is contained equally in both and the waves can do work if not -- then what the heck have I been listening to all this time on my radio ??????????
 
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  • #31
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Magnetic fields ain't like a hammer hitting a nail since the hammer does work.

Magnetic fields are like the normal force on a ramp. It doesn't do any work since it's perpendicular to the displacement.
 
  • #32
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time is important.

to EJavier.
Then please explain radiation where both fields are normal to displacement but obviously do work (equally) per the photon. They do work BUT dissappear because the stored energy is used . A purely static and unchanging field does no work , a changing field gives or accepts energy,- like a spring, an accellerating field gives rise to radiation , all of which you could equally say about the electrostatic field.
In a tuned circuit comprising a capacitor and an inductor which is oscillating energy is continually exchanged from one to the other in both potential and kinetic forms -- the introduction of a resistance causes an exponential decrease in oscillation as energy is used -- both fields decrease as work is done producing heat. The cap' and ind' voltages are both normal to the current( in the sinusoidal sense) so they produce no internal heat but they have changing energy, represented by the resistance heat.
But I agree that that is a non-conservative system in that heat is a random motion.
However the photon does not have to produce heat it could release (say) electrons from a metal surface exchanging energy to kinetic of the electron.
The normal force on a ramp does work -- it compresses the object which is stored energy - which can be released if the object falls off the ramp.
 

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